1. Field of the Invention
This invention relates to a trouble releasing system for an ATM (Asynchronous Transfer Mode) logic IP (Internet Protocol) network, and more particularly to a trouble releasing system for an ATM logic IP network which is constructed in an ATM network wherein a plurality of ATM switches and ATM terminals are connected arbitrarily and which uses an NHRP (Next Hop Address Resolution Protocol) as an address solution system between the ATM switches and the ATM terminals.
2. Description of the Related Art
In order to perform communication, in a logic IP subnetwork constructed in an ATM network, between an arbitrary ATM switch and ATM terminal, IP addresses of the ATM switch and the ATM terminal must be known.
One of such address solution methods employs an NHRP. The NHRP is an Internet draft proposed by the IETF (Internet Engineering Task Force) and is designated as &lt;draft-ietf-rolc-nhrp-10.txt&gt;.
The NHRP stores, for each logic IP subnetwork, ATM addresses and IP addresses of all of ATM switches and ATM terminals included in the logic IP subnetwork in a paired relationship with each other, and installs a server (NHS: NHRP server) for managing the ATM addresses and the IP addresses. Thereafter, the NHRP installs, for the ATM switches and the ATM terminals included in the logic IP subnetwork, clients (NHC: NHRP clients) which communicate with the NHS. Then, when any of the ATM switches and the ATM terminals tries to communicate with another ATM switch or ATM terminal, the NHC of the NHRP inquires the NHS to acquire address information to allow such communication.
Here, if the NHS which manages the addresses of the logic IP subnetwork suffers from a trouble or the like and cannot communicate any more, then the address information cannot be solved any more. This sometimes results in failure in communication with all of the ATM switches and the ATM terminals included in the logic IP subnetwork.
An example of a technique which solves a trouble of a network which arises from such failure in operation of an address server is a "Network Connection Apparatus" disclosed in Japanese Patent Laid-Open Application No. 120,950/94.
In the network connection apparatus mentioned, a router which interconnects different LANs (Local Area Networks) additionally has a function of backing up information of a server into an alternative server, another function of supervising the state of the server and a further function of holding address information of the alternative server. The network connection apparatus uses, if the server fails, alternative addresses registered in the router in advance and serving as the backup information of the server to maintain communication and avoid the trouble.
However, the prior art described above has the following problems.
First, in order to realize an end-to-end seamless connection aimed by an original ATM network and avoid a bottleneck of a drop of the transfer rate by a router, recent ATM networks which employ a system wherein a plurality of LANs are connected directly to each other without the intervention of a router are gradually increasing.
Therefore, the concept of a logic IP network has become required. Consequently, the prior art network connection apparatus disclosed in Japanese Patent Laid-Open Application NO. 120,950/94 which requires a router has a problem in that it cannot be applied to such recent ATM networks.
Second, the network connection apparatus disclosed in the document mentioned above is directed to application only to a LAN of the broadcast type which commonly has communication media such as, for example, the Ethernet.
However, with an ATM network which is an NBMA (Non-Broadcast Multi-Access) network, since it is so constructed that a connection is established when communication is to be performed, even if another address is given, communication is not performed immediately. For this communication, address solution on the logic IP subnetwork is required. However, in order to realize this, taking such a construction of a server and a client as described above into consideration, releasing means of both of them must be controlled.
In this manner, the technique of the prior art is disadvantageous in that it cannot take a countermeasure against a trouble of a server of the address solution system of a logic IP subnetwork.